Secretary of State John Kerry and national intelligence advisers arrive on Capitol Hill in Washington on Friday to update members of the House on Syria’s alleged use of poisonous gas in its ongoing civil war.

BEIRUT — The instances in which chemical weapons are alleged to have been used in Syria were purportedly small in scale: nothing along the lines of Saddam Hussein’s 1988 attack in Kurdish Iraq that killed thousands.

That raises the question of who would stand to gain as President Bashar Assad’s regime and the opposition trade blame for the alleged attacks, and proof remains elusive.

Analysts say the answer could lie in the past — the regime has a pattern of gradually introducing a weapon to the conflict to test the international community’s response.

The U.S. said last week that intelligence indicates the Syrian military has likely used sarin, a deadly nerve agent, on at least two occasions in the civil war, echoing similar assessments from Israel, France and Britain. Syria’s rebels accuse the regime of firing chemical weapons on at least four occasions, while the government denies the charges and says opposition fighters have used chemical agents in a bid to frame it.

But using chemical weapons to try to force foreign intervention would be a huge gamble for the opposition, and one that could easily backfire. It would undoubtedly taint the rebellion in the eyes of the international community and seriously strain its credibility.

Mustafa Alani, an analyst at the Gulf Research Center in Geneva, said it would also be difficult for the rebels to successfully employ chemical agents.

“It’s very difficult to weaponize chemical weapons,” he said. “It needs a special warhead, for the artillery a special fuse.”

In the chaos of Syria’s civil war, pinning down definitive proof on the alleged use of weapons of mass destruction is a tricky task with high stakes. President Barack Obama has said any use of chemical arms — or the transfer of stockpiles to terrorists — would cross a “red line” and carry “enormous consequences.”

Already, the White House’s announcement that the Syrian regime appears to have used chemical arms has ratcheted up the pressure on Obama to move forcefully. He has sought to temper expectations of a quick U.S. response, saying too little is known about the alleged attacks to take action now.

Analysts suggest that a limited introduction of the weapons, with little ostensible military gain, could be an attempt by the Syrian government to test the West’s resolve while retaining the veil of plausible deniability. This approach would also allow foreign powers eager to avoid a costly intervention in Syria to remain on the sidelines, while at the same time opening the door for the regime to use the weapons down the road.

“If it’s testing the water, and we’re going to turn a blind eye, it could be used widely, repeatedly,” Alani said. “If you are silent once, you will be silent twice.”

The slow introduction of a weapon to gauge the West’s response fits a pattern of behavior the Assad regime has demonstrated since the uprising began in March 2011, according to Joseph Holliday, a Syria analyst at the Washington-based Institute for the Study of War.

When largely peaceful protesters initially took to the streets, the regime responded with small arms fire and a wave of arrests. As the government ramped up its violent crackdown, the opposition began to take up arms in late 2011, prompting yet another escalation in force by the regime.

In early 2012, government troops began using heavy weapons, first in a relatively restrained manner on military targets.

“Once they could confirm that there wasn’t going to be a major reaction from the West, they were able to expand the use of artillery,” Holliday said.

By the summer of 2012, government troops were pounding rebellious neighborhoods with tank fire, field cannons and mortars, but the rebellion was stronger than ever, prompting Assad to turn to his air force, and the regime’s MiG fighter jets and helicopter gunships began to strike military targets in rural areas.

After the government was satisfied that the international community wasn’t going to impose a no-fly zone like NATO did in Libya, Assad unleashed the full might of his air power, and warplanes have been indiscriminately bombing rebel-held areas since.

“It all fits the pattern of being able to do this incrementally,” Holliday said.

“It’s been important for the regime to introduce these capabilities as gradually as possible so that they don’t trip the international community’s red lines,” he added. “I think this is basically a modus operandi that the Assad regime has established and tested with the United States, and confirmed that it works, and he’s using it again with chemical weapons.”

Syria has never confirmed it even has chemical weapons. But it is believed to possess substantial stockpiles of mustard gas and a range of nerve agents, including sarin, a highly toxic substance that can suffocate its victims by paralyzing muscles around their lungs.

Concern rose last summer when then-Foreign Ministry spokesman Jihad Makdissi told a news conference that Damascus would only use chemical or biological weapons in case of foreign attack, not against its own people. The ministry then tried to blur the issue, saying it had never acknowledged having such arms.

Weapons of mass destruction are generally viewed as a deterrent against foreign attack, and their use a sign of desperation. But Assad appears far from desperate at the moment, and in fact is operating from a position of relative strength.

While much of northern Syria has fallen to the rebels, the government’s hold on Damascus is firm and its forces have been on the offensive in the capital’s suburbs and in the countryside near the border with Lebanon. In the northwest, regime troops recently opened up a key supply road to soldiers fighting in the embattled city of Aleppo.

Two of the alleged attacks the Syrian opposition blames on the regime took place in and around Aleppo: one in Khan al-Assal west of the city on March 19, and another in the contested Shiekh Maqsoud neighborhood on April 13. The other alleged instances were in the central city of Homs on Dec. 23 and in the village of Otaybah outside Damascus on March 19.

It is not clear exactly how many people died in those attacks because of the scarcity of credible information. The Syrian government seals off areas it controls to journalists and outside observers, making details of the attacks sketchy. But reports from anti-Assad activists and the government provide a basic outline.

Opposition activists have posted videos and pictures online of alleged victims of the attacks foaming at the mouth or with blister burns — symptoms consistent with chemical weapons attacks, but also other munitions. The Syrian state news agency, after one attack it blamed on rebels, published photos of casualties, including children. None showed signs of physical injuries.

Both sides in the civil war, which has already killed more than 70,000 people, have tried to use the issue to sway international opinion.

Rebels have been clamoring for more robust international action against the Assad regime. At a recent gathering in Turkey of the rebellion’s international supporters, the opposition political leadership demanded drone strikes on regime targets and the imposition of a no-fly zone, and it reiterated calls for transfers of heavier weapons to its fighters.

The regime has seized on the opposition’s demands for outside support to bolster its argument that rebels may have used chemical weapons to frame the government and precipitate foreign intervention.

In December, after rebels captured a chlorine factory in Aleppo, the government warned the opposition could be planning a chemical attack to frame the regime. To back up its assertions, the state news agency pointed to internet videos that purported to show regime opponents experimenting with poisons on mice and rabbits.

In the video, a masked man mixes gases in a glass box containing two rabbits. About a minute later, the animals start to spasm and then collapse. A narrator then says, “This is what will happen to you, Assad supporters.” The origin of the video was not known.

Alani dismissed the possibility of the rebels, including Islamic extremist groups among the most powerful opposition fighting factions, carrying out a chlorine attack.

He noted that al-Qaida militants used chlorine on at least two occasions in Iraq in the cities of Ramadi and Fallujah, but abandoned the practice because “the impact of the chlorine was far less than conventional explosives.”